1. Field of the Invention
The present invention relates generally to impellers for use in pumping systems, compressors and the like and, in particular, to a precision milled impeller having a cover member, a hub member and multiple blades extending therebetween.
2. Description of Related Art
Impellers can be used in many different applications. For example, impellers could be used in connection with a blower, a compressor, a centrifugal pump or compressor, generators, water pumps, transmission mechanisms, refrigerant compressors, etc. While the design of the impeller in most applications is generally similar, there are many internal and external factors that should be considered when precisely designing and manufacturing an impeller for any of the different applications, such as for a centrifugal compressor which requires increased integrity of the critical flow path during operation. In addition, in such an application, the blades or vanes must have sufficient rigidity and minimal stress and other failure points.
Generally, impellers come in one-piece, two-piece or three-piece designs. For example, as seen in U.S. Pat. No. 6,146,094 to Obana et al., an impeller according to the prior art is illustrated. As best seen in FIG. 1 of the Obana patent, the impeller includes a front plate 101 and blades 103. The front plate 101 and the blades 103 are formed as a monolithic structure from a single piece of material. The front plate 101 includes a suction opening 801, and a separate back plate 102 is attached to the structure opposite the front plate 101 and blades 103. Accordingly, the blades 103 are sandwiched between the front plate 101 and the back plate 102. The blades 103 of the Obana patent are curved in what is commonly referred to as a three-axis structure or arrangement.
The front plate 101, the back plate 102 and the blades 103 define multiple air outlets 802, such that when the impeller is rotated, air is sucked through the suction opening 80 into the impeller, and is discharged through the air outlets 802 toward, in this case, an electric motor, in order to cool the motor. In one embodiment, the front plate 101 and the blades 103 are formed in a monolithic structure by an injection molding process, and the back plate 102 is bonded to the blades 103 along the entire length of the blades by brazing. Specifically, an inner surface of the back plate 102 is coated with a brazing metal layer 201, and the front plate 101, which is integral with the blades 103, are held against the back plate 102 and heat is applied such that the back plate 102 bonds to the blades 103 by brazing.
Another impeller design is described in Patent Application Publication No. US 2003/0133801 to Orocio et al. As with the impeller of the Obana patent, the impeller of the Orocio reference is also integrally formed and molded in a single operation. The impeller of the Orocio reference is designed for use in connection with a centrifugal pump, where rotation of the impeller causes liquid supplied to the inlet or center area of the impeller to be radially accelerated and dispensed from the periphery of the impeller.
The impeller of the Orocio reference includes a shroud 36 with an annular inlet ring 38 centrally formed with the shroud 36. Mounted on the inside surface of the shroud 36 are multiple vanes 42, which are, as discussed above, of a three-axis design and structure. A cover 46 is placed about the hub 44 and is welded or otherwise secured to the vanes 42. Liquid is passed through passage 29 of the inlet conduit 30 and deposited within eye 48 of the pump impeller 34. By rotating the pump impeller 34, liquid is drawn from passage 29 into enclosing chamber 37 and propelled radially outwardly past peripheral edge 35 through vane openings 33 into discharge passes 31 of the discharge conduit 28. The pump impeller 34 of the Orocio reference is molded in a one-step molding operation.
These prior art impellers have many drawbacks. For example, the impeller of the Obana patent would be considered a two-piece impeller, where the front plate 101 and the blades 103 are formed as a monolithic structure, and the back plate 102 subsequently attached thereto. While the impeller of the Orocio reference discusses creating the shroud, vanes and shaft sleeve as a single structure for alignment purposes, as with the impeller of the Obana patent, the impeller of the Orocio reference is a molded piece. Such molded pieces exhibit many deficiencies and cannot be used in various high-end applications where greater precision is required. In particular, such molded pieces are “sloppy” and exhibit limited output when used in high volume applications. In addition, neither of the impellers of the Obana patent nor the Orocio reference are formed with precision blades or vanes of the five-axis type, as such blades, which make a 90-degree turn from leading edge to opposing edge, would not exhibit appropriate stabilization characteristics and could not be manufactured in a molding process to achieve such characteristics.
Impellers for high-speed compressor applications, such as multi-stage centrifugal compressors, are, as discussed above, in the form of two circular disks separated by and sandwiching radially extending vanes or blades. These vanes define spiral passageways between the disks that form the impeller, and fluids in the passageways are directed outward toward the outer periphery of the disks as the disks rotate. When forming such impellers for high volume or high-speed applications, and when forming these impellers from various metallic or metal-based materials, the fabrication process according to prior art exhibits many deficiencies.
According to the prior art, furnace brazing is utilized, where one disk or cover is provided with integrally-formed vanes extending from the surface, and this disk is manufactured by a machine by milling the disk and vanes from a single piece of metal. Next, the back cover is attached in a brazing process, where a surface of the cover to be attached includes a material with a melting point below that of the disks and vanes. The impeller is placed in a furnace, and as the impeller components heat, the brazing material melts and the disks are forced together. Oftentimes, excess brazing material forms a ridge or other obstruction in the fluid passageways of the impeller. In addition, the impellers exhibit some residual internal stresses due to the expansion, braze solidification and subsequent uneven contraction of the parts. Further, as discussed above, the disruptions of the brazing material disrupts the characteristics of the impeller in the fluid passage therein. The strength of the brazed joints is much less than the strength of the material it joins, and therefore fatigue and fractures occur.